1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, and particularly to a control system for an internal combustion engine having an exhaust gas recirculation device for recirculating a part of exhaust gases to an intake system of the engine.
2. Description of the Related Art
Japanese Patent Laid Open No. 2006-250120 (JP'120) discloses a fuel injection control apparatus for a diesel engine. According to this apparatus, a main fuel injection is performed in one combustion cycle with a plurality of divided injections (e.g., two injections), and a dividing ratio of amounts of fuel injected in the divided fuel injections (e.g., a ratio of the fuel amount of the first injection to the fuel amount of the second injection) is changed according to the engine load. With this method, deterioration of the fuel consumption rate and an increase in an amount of generated soot are suppressed when enriching the air-fuel ratio.
Further, Japanese Patent Laid Open No. 2006-214322 (JP'322) discloses a control system for an internal combustion engine having a lean NOx catalyst in the exhaust system. Since NOx is trapped in the lean NOx catalyst during a lean operation wherein the air-fuel ratio is set to a comparatively large value, it is necessary to timely supply reducing components (HC, CO) to the lean NOx catalyst so as to reduce the trapped NOx. In the system shown in JP'322, an amount of NOx trapped in the lean NOx catalyst is estimated according to an engine rotational speed and a demand torque of the engine. The NOx amount in the feed gas (gas discharged from the combustion chamber of the engine) changes depending on an exhaust gas recirculation amount (an amount of exhaust gases recirculated to the intake system). Therefore, in the system of JP'322, the amount of trapped NOx is corrected according to the exhaust gas recirculation amount.
In recent years, the demand for the diesel engine is increasing for reducing carbon-dioxide emission. On the other hand, since the regulation of the NOx emission amount is very strict especially in the United States, it is necessary to surely maintain the NOx emission amount below the regulation value for a long period of time. Therefore, it is necessary to significantly reduce the NOx emission amount of the diesel engine, and also to improve robustness of the NOx emission amount against characteristic variation in mass-produced engines (hereinafter referred to simply as “characteristic variation”) or aging. The robustness means a characteristic of being hard to be influenced by the characteristic variation or aging.
As shown in JP'120, a setting of the dividing ratio when performing the divided fuel injections influences the NOx emission amount. In the apparatus described in JP'120, the dividing ratio is merely set according to the engine load. Accordingly, the robustness of the NOx emission amount cannot be improved.
Since the NOx amount in the feed gas changes depending on the exhaust gas recirculation amount, it is effective for improving the robustness to detect a deviation amount of the exhaust gas recirculation amount due to the characteristic variation or aging. However, it is difficult to accurately estimate the exhaust gas recirculation amount, using maps or tables which are previously set to calculate the exhaust gas recirculation amount, or using a physical model of the engine, since the exhaust gas recirculation amount is influenced by various factors, such as the boost pressure, the fuel injection amount, and the intake pressure.
Therefore, even if an exhaust gas recirculation amount sensor is provided to detect the exhaust gas recirculation amount, it is difficult to determine whether or not the detected value is a proper value, or whether or not the detected value is obtained in a normal (typical) condition of the exhaust gas recirculation device. Consequently, the dividing ratio of the fuel injection amounts cannot be set to an appropriate value, to improve the robustness of the NOx emission amount.
Further, in the system shown in JP'322, a deviation between a detected intake air flow rate and a target intake air flow rate is used as an exhaust gas recirculation amount parameter indicative of an actual exhaust gas recirculation amount, and a correction of the trapped NOx amount is performed according to the exhaust gas recirculation amount parameter. However, the exhaust gas recirculation amount is influenced by various factors, such as the boost pressure, the fuel injection amount, and the intake pressure. Therefore, it is difficult to estimate the actual exhaust gas recirculation amount using the maps or the tables which are previously set to calculate the exhaust gas recirculation amount, or using the physical model, even if the engine is operating in a steady operating condition. In a transient operating condition of the engine, the difficulty further increases. Therefore, the exhaust gas recirculation amount parameter shown in JP'322 does not accurately indicate the actual exhaust gas recirculation amount, and the correction cannot be performed accurately.
Particularly when controlling a diesel engine, a closed-loop control is performed wherein the exhaust gas recirculation amount is made to coincide with a target value based on the air-fuel ratio detected by an air-fuel ratio sensor, a detected value of the intake air flow rate by an air flow meter and the like, and/or another closed-loop control is performed wherein a vane opening of a turbocharger is adjusted so that the boost pressure coincides with a target pressure. Therefore, the control systems performing the above closed-loop controls intricately cooperate with each other, to accordingly compensate the characteristic variation or aging changes in the exhaust gas recirculation amount. Therefore, an accurate trapped NOx amount cannot be obtained by the simple method as shown JP'322, and the timing of performing a process for reducing NOx may be inappropriate, which may increase the NOx emission amount.